The motor vehicle market in particular, is highly dependent on this kind of steel sheet which is coated on at least one face and is generally referred to as "galvanized", and this is above all because of its corrosion resistance which enables it to be used in this form. It also happens that it is intended to be painted, in which case a phosphating pretreatment is found to be necessary to ensure good paint adhesion.
Although the invention may be applied generally to monolithic products, such as aluminum sheets, for the sake of simplicity the following will be directed to only steel sheets coated with a layer of a compound of the "zinc-aluminum" type, this, being doubtless the most common case.
It is known that these aluminous compounds comprise, on the surface, a thin film of oxide of these metals, and particularly of alumina, which forms naturally. The presence of these oxides on the surface interferes with the subsequent surface treatment operations to which the sheet metal may be subjected, and especially the phosphating operation.
It has been found, in particular, that it was difficult to obtain a correct phosphating, and sometimes even that phosphating was impossible, when the aluminum content of the coating was high.
For example, it has been possible to establish that steel sheet coated with a Zn-Al alloy containing 5% of aluminum could be phosphated correctly only if fluoride ions were added to the phosphating bath (cf. French patent Application No. 2,575,188).
In the case of a zinc-aluminum coating with a higher content of aluminium, for example of the order of 55% and above, phosphating was not being produced at all in a bath free from fluorine and, in a bath which contained it, the phosphate layer formed on the aluminum-rich coating was highly heterogeneous and porous.
French Patent Application No. 2,567,158 describes a process for the preparation of a steel product galvanized hot by dipping in a bath containing zinc or a Zn-Al alloy, and intended to be phosphated. This process consists in forming on the product an electrolytic deposit of a metal chosen from a specific group of metals, it being possible for this metal, in particular, to be zinc. According to this document, the zinc layer deposited in this manner on the film of oxides which was formed on the surface of the coating makes it possible to ensure better anchoring of the phosphate layer which is subsequently deposited.
However, from an industrial standpoint, the process of electrolytic deposition in an acidic medium, described below, has the disadvantage of requiring a modification of the conventional phosphating line assembly, because of the introduction of an additional treatment involving the use of an electrolysis vessel, and of its electrical supply.
What is more, as indicated in the above-mentioned document, the electrolytic deposition is preceded by an electrolytic pickling (electrochemical polishing) bath which is necessary to remove the oxide layer formed on the surface without contaminating the zinc deposition bath. It will readily be understood that, because of the two electrolytic baths, this process is a particularly high consumer of electrical energy, and this represents a major disadvantage from the standpoint of economics.
British Patent Application No. 2,152,955, also describes a process for the treatment of metal sheets coated with a zinc-aluminum alloy with a view to facilitating a subsequent phosphating operation. According to this process, the metal sheet is treated in an alkaline solution intended to strip off the aluminum present on the surface. However, as the examples described show, the coating of the treated metal sheets is relatively poor in aluminum (less than 10%).
Furthermore, the solution employed contains ions of at least one of the following metals: nickel, iron, cobalt and manganese. These metals can deposit on the surface of the coating and, although they tend to facilitate the subsequent phosphating, they form a heterogeneous surface layer with the zinc of the coating. Electrochemical phenomena (cell phenomena between different metals) can be produced within this layer, leading to a subsequent deterioration of the latter.